This disclosure relates to media handling within a printing apparatus, and more particularly, to a calibration and diagnostic system for use within the paper path of a printing apparatus to determine media speed and skew.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the information areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet. Such an electrophotographic printing process is shown in U.S. Pat. No. 6,137,989, which is incorporated herein by reference.
Typically, in printing machines described above, it is necessary to place the image with some precision on each sheet. This requires the ability to register a sheet with respect to the transfer station. Sheet registration of the top edge or inboard to outboard edge of the print, can be achieved in several ways. Edge registration, such as with crossed rolls, registers the top edge of the sheet by moving the sheet in a diagonal direction so that it eventually contacts against a side registration edge. The sheet is then transported forward in the process direction by servomotors that bring the sheet to transfer at the right time and at the right velocity. Translating electronic registration schemes utilize two or three stepper motors, which deskew the lead edge and simultaneously register the top edge of the sheet and then deliver the sheet at the right velocity and right time to transfer. The most inexpensive registration method is stalled roll deskew, whereby the sheet lead edge travels into a non-moving or stalled roll nip. The body of the sheet continues to move forward, pushing the lead edge forward so that it aligns eventually with the nip. When the stalled nip is activated, such as by an elector-mechanical clutch, the lead edge of the sheet, now deskewed, moves forward, and the body follows naturally.
The cost of these three systems has been estimated at approximately several hundred dollars for crossed rolls with servo or the translating electronic registration system with stepper motors, and nearly one hundred dollars for the stalled roll registration system. The stalled roll registration system is by far the most economical and is also the most frequently used registration system in mid to low cost printer/copiers. The stalled roll registration system, however, does not correct for top edge misregistration.
U.S. Pat. No. 4,519,700 issued May 28, 1985 to Barker et al. describes a xerographic image transfer device in which copy sheets are sequentially aligned and position sensed before introduction to the image transfer zone. The position sensing is used to compare the copy sheet location with the position of the image panel on a moving phototconductor. The timing and velocity profile of the copy sheet drive after the position sensing is arranged so that the copy sheet arrives in registry with the image panel and at the same velocity.
A sheet feeding and lateral registration system is shown in U.S. Pat. No. 5,273,274 issued Dec. 28, 1993 that includes feed rollers for feeding sheets in a process direction and registration apparatus for registering each sheet in a direction laterally of the process direction. The registration apparatus includes a shifting system for laterally shifting a carriage on which the feed rollers are mounted. A single edge sensor is arranged to provide a signal on detecting the presence of a sheet, and a controller controls the lateral shifting system in response to that signal. The controller is operated such that if the sheet is not detected by the sensor on initial entry of the sheet into the feed rollers, then the shifting system is activated to move the feed rollers laterally towards the sensor until the sheet is detected by the sensor, whereupon the lateral movement is stopped. If the sheet is detected by the sensor on initial entry of the sheet into the system, then the shifting system is activated to move the feed rollers laterally away from the sensor until the sensor no longer detects the sheet, and then the shifting system is reverse activated to laterally move the feed rollers back towards the sensor until the sheet is again detected by the sensor.
U.S. Pat. No. 5,678,159 issued Oct. 14, 1997 to Williams et al. discloses a deskewing and registration device for an electrophotographic printing machine. A single set of sensors determines the position and skew of a sheet in a paper path and generate signals indicative thereof. A pair of independently driven nips forwards the sheet to a registration position in skew and at the proper time based on signals from a controller which interprets the position signals and generates the motor control signals. An additional set of sensors can be used at the registration positions to provide feedback for updating the control signals as wear or different substrates having different coefficients of friction are used.
An apparatus and method for correcting top edge sheet misregistration using a sensor array is disclosed in U.S. Pat. No. 6,137,989 issued Oct. 24, 2000 to Lisbeth S. Quesnel. An array sensor is placed in the paper path prior to transfer. A signal is generated indicating the position of the sheet. As a function of the signal, the print controller causes the image to be exposed and developed on the photoreceptor in alignment with the sheet position. The aligned image is then transferred to the sheet.
U.S. Pat. Nos. 6,168,153 B1 and 6,173,952 B1 issued Jan. 2, 2001 and Jan. 16, 2001, respectively, to Paul N. Richards et al. disclose a sheet handling system for correcting the skew and/or transverse position of sequential sheets, especially those moving in a process direction in a sheet transport of a reproduction apparatus. The system employs sensor arrays in deskewing and/or side registering sheets.
Some of these technologies are quite sophisticated, utilizing various embedded sensors, digitally controllable stepper motors and high speed computational capability, all of which add up to a significant level of equipment cost, which, while justified in a high-end printer, might be considered exorbitant in a smaller, less expensive device.
Even though the above-mentioned prior art is useful, there is still a need, in printer/copiers for a more cost-effective paper path skew correction system.
Accordingly, a sheet to sheet electronic skew correction system is disclosed that answers the above-mentioned problem by measuring the input skew of each incoming sheet “on-the-fly” with sensors and then adjusting the image electronically to match the skew of the particular sheet. Thus, the image placed on the sheet from the photoreceptor is square with the dimensions of the sheet since the image is an exact match with the skew orientation of the sheet. The sheet can then be transported by a final nip that corrects only the lead edge timing.
While the disclosure will be described hereinafter in connection with a preferred embodiment thereof, it will be understood that limiting the disclosure to that embodiment is not intended. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.